This invention relates to optical (de)multiplexing, and in particular to wavelength stabilized planar waveguide multiplexers and demultiplexers and optical components incorporating these devices.
Integrated wavelength demultiplexers (either phased waveguide arrays or grating-on-a-chip spectrometers) are important components for wavelength division multiplexing (WDM) optical communication systems. Integration offers the advantages of compactness, reliability, and reduced packaging costs. Further, implementation in a semiconductor material, particularly the InGaAsP/InP system, which is important for optical fiber communications systems, permits the monolithic integration of these passive devices with active ones, such as lasers, modulators, optical switches, and detectors, resulting in sophisticated wavelength sensitive photonic integrated circuits with complex functionalities. A grating demultiplexer is described in Appl. Phys. Lett. 58, 1949 (1991), J. B. D. Soole et al., xe2x80x9cMonolithic InP/InGaAsP/InP grating spectrometer for the 1.48-1.56 xcexcm rangexe2x80x9d.
One of the major issues in integrated wavelength demultiplexers (DEMUX) is control of the absolute wavelengths of the various channels. The relative accuracy of the channels (that is, the spacing in wavelength or frequency between adjacent channels) can be accurately determined by the geometrical design of the demultiplexer and the optical properties of the waveguide. The geometrical design can be laid out with high precision using modem e-beam technology to write photolithographic masks and waveguide material uniformity over demultiplexer dimensions is adequate to ensure precise channel spacing. Provided at least one channel of the demultiplexer array is set at the correct wavelength, all of the other channels are automatically at their correct wavelengths. This is one of the main advantages of integrated demultiplexers over hybrid demultiplexers which combine many individual elements (Bragg fiber filters or interference filters, for example) in a complex hybrid package. Each element of such a hybrid demultiplexer must be fabricated individually to very tight tolerances so that absolute wavelengths arc maintained and channel spacings are accurate.
Integrated demultiplexers are generally tunable by temperature since waveguide optical properties, such as effective index, are temperature sensitive. This behaviour provides flexibility in that channel wavelengths can be adjusted by temperature control, but at the same time, uncertainty increases since the absolute wavelength of a given channel is temperature sensitive. What is required is a means to ensure that the wavelength setting for each channel is accurate, to within the tolerances set by the International Telecommunications Union (ITU) WDM grid.
At present, the most straightforward means available for establishing the accuracy of integrated demultiplexers is to calibrate each device at the factory. This is accomplished by coupling a laser of known wavelength (in the ITU grid) into the device and then adjusting the device temperature to maximize the optical signal exiting the appropriate channel. This temperature must be maintained during operation to ensure that the device remains calibrated.
Another technique requires the use of a feedback circuit to tune the DEMUX with temperature. By monitoring the intensity of a signal of known wavelength in a channel set aside as a monitor, it is possible, in principle, to lock onto that wavelength and thereby tune all of the channels of the whole DEMUX to their proper wavelengths. This requires that a special wavelength, with an accurately known wavelength, be present in the system at all times. This could also function for example, as an optical supervisory channel (OSC).
The method of calibrating an integrated demultiplexer, at the factory, and, then maintaining that wavelength accuracy in the field, as outlined above, has serious limitations. If the ambient temperature of die device changes, the cooling or heating necessary to maintain calibration will change in an unknown manner. Furthermore, aging effects, either in the waveguide material itself or in components of the packaged device (e.g., the glue or epoxy used to hold the demultiplexer onto the temperature maintaining clement) may change the optimum cooling or heating conditions. The temperature controller itself may undergo changes as it ages, thus altering the device temperature (and wavelength calibration) in an unknown fashion.
The technique of using a feedback mechanism is potentially more promising. However, if there are a large number of wavelengths present in the system (so-called dense WDM), the monitor channel will need to be very narrow (much less than the channel passband), and if the signal starts to drift off the correct position (e.g., by changes in the ambient temperature) or if the signal intensity varies due to changes in the network or aging, the feedback circuit finds it difficult to adjust quickly or accurately. A constant dithering would probably be required to maintain lock-on. Furthermore, if the signal were lost altogether, the circuit would have no way of knowing in which direction to look for the signal, increased or decreased temperature . While seeking to recover the signal, all of the wavelengths in the array would be out of specifications. Further, the narrow passband required for high-density WDM would make it difficult to lock onto the signal easily.
One attempt to overcome this problem is described in the paper entitled xe2x80x9cA Wavelength Matching Scheme for Multiwavelength Optical Links and Networks using Grating Demultiplexersxe2x80x9d, Photonics Technology Letters Vol. 7, No. 6, June 1995, F. Tong et al.
In a conventional grating demultiplexer, incoming light confined in a planar waveguide is diffracted off an echelon grating and focussed onto regions forming waveguide channels in the planar waveguide below ridges formed on the surface of the device. Typically, the ridges should be far enough apart to avoid overlap between the channels, which would cause crosstalk. In the technique described in the Tong article, the separation between two of the output channels is reduced and a reference wavelength is focussed on to a region between two adjacent channels so as to partially overlap them both. This signal is picked up by both channels, and as drift occurs the proportion of the signals received by the respective channels changes. Typically the channels are 4 xc3x85 apart. The problem with the Tong proposal is that the device can easily drift by 8 to 12 xc3x85 or more, which can result in total loss of signal, in which case the Tong proposal offers no improvement over a simple feedback arrangement. Tong does not have the ability to follow deviations exceeding a very narrow range and possibly for this reason appears not to have been put into practical use.
An object of the invention is to alleviate the afore-mentioned problems of the prior art.
According to a first aspect of the present invention there is provided a wavelength stabilized planar waveguide optical device comprising an input channel, a plurality of output data channels, and a dispersive element for directing component wavelengths of incoming light into said respective output channels, a pair of monitor channels for receiving a reference wavelength having a separation less than, and a width greater than, said data channels, and a differential feedback arrangement for controlling the temperature of the demultiplexer according a differential signal received from said pair of monitor channels to stabilize the wavelengths of the device.
The separation refers to the distance between the edges of the channels, and in the preferred embodiment can actually be zero; i.e. the channels touch at their inputs.
Only one pair of monitor channels is required, although additional monitor channels forming additional differential pairs could be provided if desired. The monitor channels are preferably in contact at their inputs, and a control wavelength, possibly also providing a supervisory channel, is directed to a point where it overlaps the monitor channels equally when the device is locked to the control wavelength. In a preferred arrangement, the monitor channels taper outwardly in width toward their inputs, where they are in contact with each other, and the monitor wavelength is directed by the dispersive element so as to extend equally on either side of the line of contact between the two channels.
By widening the monitor channels relative to the data channels, it is possible to maintain a lock on the signal even when the device drifts by as much as 8 to 12 xc3x85.
A similar result can be achieved by defocussing the reference beam at the inputs to the monitor channels. Accordingly, a second aspect of the invention provides a wavelength stabilized planar waveguide optical device comprising an input channel, a plurality of output data channels, and a dispersive element for directing component wavelengths of incoming light into said respective output channels, a pair of monitor channels for receiving a reference wavelength, said monitor channels having inputs positioned relative to said dispersive element such that said reference wavelength is defocussed at the inputs to said monitor channels to increase its spot size, and a differential feedback arrangement for controlling the temperature of the demultiplexer according a differential signal received from said pair of monitor channels to stabilize the wavelengths of the device.
The monitor channels are conveniently separated from the data channels so as to avoid taking up data bandwidth.
Normally, the demultiplexed wavelengths are tightly focussed on to the output data channels to achieve maximum resolution and minimum crosstalk. The effectiveness of the monitoring device can be further enhanced by defocussing the monitor wavelength so as to broaden the spot in the vicinity of the monitor channels. In this way, because the light is spread over a greater area, the monitor channels are more tolerant of large deviations from normal.
The invention also provides a method of wavelength stabilizing a planar waveguide optical device incorporating a dispersive element comprising the steps of receiving multiplexed light containing a reference wavelength on an input channel; directing component wavelengths of the incoming light into respective data channels and said reference wavelength onto a pair of monitor channels with the aid of said dispersive element, said monitor channels having a separation less than, and a width greater than, said data channels, and controlling the temperature of the demultiplexer with a differential signal received from said pair of monitor channels to stabilize the wavelengths of said device.
The invention is applicable to both phased waveguide arrayed and diffraction grating demultiplexers.